Fixed #5 by getting offset and outerHeight

Fixed https://github.com/Eonasdan/bootstrap-datetimepicker/issues/5.  I tried to run the tests but kept running into errors, the last of which was:

$ make test
mkdir -p build/js
mkdir -p build/css
aborting
Fontconfig warning: "/etc/fonts/conf.d/50-user.conf", line 9: reading configurations from ~/.fonts.conf is deprecated.
TypeError: 'undefined' is not a function (evaluating 'mocha.ui('bdd')')

  file:///home/tim/Documents/MyWebPages/arwd/projects/backbone/_attachments/js/vendor/bootstrap-datetimepicker/test/test.html:25
make: *** [test] Error 1